My school coilgun poject / Electromagnetic Projectile Accelerators / Forums

Forums

4hv.org :: Forums :: Electromagnetic Projectile Accelerators

« Previous topic | Next topic »

My school coilgun poject

first previous 2 3 4 5 next last

Move Thread

LAN_403

Skynet

Wed May 27 2015, 08:30PM

Registered Member #11734 Joined: Thu Mar 21 2013, 08:44PM
Location: Brno Czech republic
Posts: 35

So....
Today I finaly tested halfbridge prototype and it works quite well. But I can only runs at 200V because my drivers have saturation protection set to 300A.
Next step would be adding current trasducer for current regulation that could be achieved with halfbridge ^^

Here is vid from testing:

Skynet

Wed Mar 07 2018, 05:30PM

Registered Member #11734 Joined: Thu Mar 21 2013, 08:44PM
Location: Brno Czech republic
Posts: 35

Well after long frustration i've decided to take this project back on road :D I have managed to redesign gate drivers and completely redesign power stage. It would be multistage too (hopefully 3-5 stages).

There are some prohos of progress and more to come!

1520443844 11734 FT158837 Wp 20180307 10 30 10 Pro

1520443844 11734 FT158837 Wp 20180306 21 59 37 Pro

DerAlbi

Wed Mar 07 2018, 11:57PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Thats some development! The PCBs look good - its nice to see that someone puts some effort into it

But i dont understand how the driver works.. can you share the schematic and IGBT model number?

Skynet

Fri Mar 09 2018, 07:04PM

Registered Member #11734 Joined: Thu Mar 21 2013, 08:44PM
Location: Brno Czech republic
Posts: 35

Yup for sure ;) Its just optocoupled GATE driver with saturation protection ACPL337J :) And IGBT module is CM400DU-12F but I use only one IGBT from each module.

DerAlbi

Fri Mar 09 2018, 10:16PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Thanks!
Just some feedback: (your circuit is fine, no worries)
1) I had the idea of desaturation detection myself in an earlier revision of a driver. The issue is that its just expensive, complex and i am not sure that it actually will protect anything. See, in a purely inductive load the current is anyway limited due to ESR most likely below any current your IGBT will desaturate at. This kind of protection is only good to avoid a dead short circuit and to actually help under those conditions (all the inductive peaks and what not) it must be designed really well. Not only the driver but also the overall build.
2) Your driver can deliver 4A without the external transistors - this should be enough. You do not need and (should not have) excessive fast switching.
3) Your IGBTs are way too big ^_^ If they ever die, think about replacing them with smaller TO247 IGBTs. I have some IXGK320N60A3 laying around.. if you want some... they have basically the same ratings, but of course cant dissipate that much heat, which is not necessary anyway.

As far as i can see your setup is built as a asym. halfbridge.. Very good!
How do you want to expand the setup for more coils?
Would you need more IGBTs or whats the plan?

Skynet

Sun Mar 11 2018, 08:45PM

Registered Member #11734 Joined: Thu Mar 21 2013, 08:44PM
Location: Brno Czech republic
Posts: 35

Hi,
about desat protection it's meant to be just a failsafe (you never know what could go wrong :D) External transistors are there because i want kinda fast switching just to have something to sort for my final thesis :D Primary for current protection would be used currnet sensor HAS 200-S from LEM. Exesive voltage spikes would be handled with snubber caps across IGBT's ;)
And yes i know that they are quite bigger than its needed but I am not trying to create small coilgun. Also it's easier to maintain wiring. I maybyle consider your's IGBT's for last 2 stages.

And expanding.... Yes I would use more IGBT's and more caps it would be quite a beast :D The settup that you see on picture would be just multiplied.

DerAlbi

Mon Mar 12 2018, 05:53PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

A lot of Questions incoming

So does it shoot already?

May i ask which dimensions your coil and projectile have?
Have you measured the velocity of your projectile?

From what i can see your projectile is around 8-10mm and maybe 40-50mm long?
Your coil length seems equal to the projectile length and the wire is around 2mm i guess. Have you measured the ESR and Inductance of such coils; i would also be interested in the weight of your coils?
Your capacitors are... 350V or 450V with which capacitance?
You use a microcontroller to time things and to evaluate the the light traps. May i ask which kind of current waveform you go for? Is it like PWM, or is it just plain turn on + hold + turn off?

What i find strange is that yi cant figure out which wire goes to what coil - it seems like 2 wires are connected to the IGBTs, but both wires come from different coils - is there anything special going on? (it seems like if you connect the 2 wires in the air, both coils would be in series -- was that your test setup?)

Sorry i ask a lot, but i really like it when some one else builds stuff that is this complex (more than the SCR design). I try to compare it to my acceleration path, maybe we can learn a thing or two from comparing 2 different implementations that seem to have sort of equal dimensions..

Skynet

Mon Mar 12 2018, 10:31PM

Registered Member #11734 Joined: Thu Mar 21 2013, 08:44PM
Location: Brno Czech republic
Posts: 35

Yup it does shoot but I did tests only on low voltage because I am waiting for snubber caps and I dont want to blow up my IGBT's by doing tests without propper protections.
Projectile and coils are 50mm long and diameter is 10mm for projectile and inner diameter for coil is 11mm.
And yes wire is 2mm in diameter :D ESR for both caps in paralel is 7mâ„¦ for coil i dont have propper equipment for measuring that low resistance. I thing that weight of coils would be affecetd with used epoxy but for now i dont have scale but when i got some i will weight them :D
Caps are 450V and 4700uF.
My current waveform shoud be on + hold + turn off. But for protection i have there current sensors for possible pwm just to reduce current that could blow my IGBT's.

And at last the second coil is not connected at all. Its just prepared to be wired up with another IGBT's :D

Nevermind you could ask everything you want. Maybye some day internet learn how to build coilgun corretly from us :D

DerAlbi

Tue Mar 13 2018, 06:13AM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Thanks for the response

Unfortunately there is too much bad information out there (mainly on youtube) so we wont change the world

Considering your snubbers:
the coil it self will oscillate when the current gets back to zero. To prevent this reaching the IGBTs, you can simply put some diodes in series. (Some beefy/cheap P600G from ebay will do, parallel them if you like. A snubber across the coil will help a lot.
If you want to protect the IGBTs from a C-E-voltage spike however, a snubber capacitor wont so the job. There are several reasons:
A true snubber does have a series resistance. Since a snubber is directly stressed with the turn-off current in a halfbridge, the ESR must be low enough to not reach the 600V limit during turn-off. Such a low ESR will however result in an extremely high turn on current. So nothing gained in the end. A RC-snubber does not work in a coilgun, not even a RCD-snubber because the diode is the bottleneck..
The problem is (i learned this "recently"), that the voltage spike on the collector is not caused by ESL, but much much more by the diode forward recovery behavior. (A diode does not become forward conductive immediately). The only way to combat this is to use active clamping:
In your case, use a much larger gate resistor (you really dont need to switch faster than your diodes can handle the resulting turn on, i use 47R as gate resistor, thats still low enough to make the IGBTs faster than my diodes) and put a TVS diode (2 in series to get 450V < V_breakdown < 600V) between collector and gate. This will conduct any excessive C-E-spice towards the gate so during turn of a gate voltage plateau forms that will keep the IGBT conductive until the half-bridge diodes are ready to take over the current from the IGBTs.
So you arent fighting parasitics like ESL but you are most likely fighting semiconductor effects in the diodes. If you think about using faster diodes - forget it. The only way around the diode forward recovery would be using schottky, but they have poor (over-)current rating, so you are bound to silicon junctions.

You really should measure this at low voltages before you try to optimize this with snubbers or what not. You have an oscilloscope! Use it

hen918

Tue Mar 13 2018, 10:25AM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

DerAlbi wrote ...

Thanks for the response

Unfortunately there is too much bad information out there (mainly on youtube) so we wont change the world

Considering your snubbers:
...
The problem is (i learned this "recently"), that the voltage spike on the collector is not caused by ESL, but much much more by the diode forward recovery behavior. (A diode does not become forward conductive immediately)...

I know this is a rather expensive solution, but have you tried one of the new SiC high voltage schottky diodes?

first previous 2 3 4 5 next last

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob